The present invention relates, in general, to a high resolution ultrasonic scanner, and more particularly to a transducer array producing an output acoustic wave pattern through the controlled excitation of the elements of the array. Most particularly, the invention is directed to biomedical transcutaneous B-scanning apparatus.
Existing B-scan probes, which are particularly useful in medical diagnostics, fall into two general categories, both of which utilize transducers capable of producing pressure waves which can be formed into a beam, the direction of which is changeable so as effectively to scan a sector. One category of probes includes those which employ a single beam transducer assembly wherein the beam can be steered either by mechanically moving the assembly or by controlling the phase of the electrical signals applied to the transducer elements to scan tissue under study. A second category includes probes which employ an array of transducer elements wherein the scanning motion is achieved by electronically addressing different portions of the array. In transducers of either category, pressure waves are radiated from the transducer elements and pass into and through tissues of interest, with a portion of the wave energy being reflected whenever the waves encounter an interface of tissues having different acoustic characteristics. The returned energy is directed to suitable receiving transducers which convert the reflected pressure waves into corresponding electrical signals for display, for example, on a cathode ray tube.
Transducers in the first category produce the best resolution of tissue configurations, since the transducer assembly can readily be optimized so that the field covered by its single beam is of the desired size for detecting specific features. However, such assemblies must either be mechanically driven for beam steering, or must utilize complex phasing networks to obtain the desired scanning motion of the beam, and such arrangements are complex and expensive. On the other hand, probes in the second category, employing an array of electronically scanned transducer elements, are simpler and considerably less expensive, although their resolution is inferior.
In its simplest form, an electronically scanned transducer array consists of a row of transducer elements. One element of this array is employed in an ultrasonic pulse-echo mode in order to measure acoustic reflections off tissue interfaces ahead of the transducer as a function of the distance from the transducer. This same procedure is repeated for all of the elements of the linear array and all of the individual lines of view are combined to provide a full picture, ideally showing the outlines of all acoustically different tissue interfaces in the object under study. In a phased transducer array, each element is individually connected to a transmitter and receiver, but adjustable delays are provided in each receiver channel in order to enhance the reception from a selected direction. By suitably controlling the time at which electrical drive signals are applied to each of the transducer elements and by controlling the adjustable delays of the separate receiver channels, the effective direction of the pressure wave beam can be steered to any desired angle within a fan-shaped sector. In practice, a plurality of radial lines within the sector are successively generated to sequentially scan the entire sector. A set of such lines is generated over a short period of time, and the received signals are displayed on a cathode ray tube to provide a visualization of the outline of the tissue interfaces which produce the received signals. This visualization is known as a B-mode display, wherein variations of the acoustical impedance of the tissues are translated into brightness variations on the cathode ray tube.
Obtaining a high resolution ultrasonic image for medical diagnostic purposes requires an ultrasonic transmit-receive system having a small focal spot-size over its entire field of view. Otherwise, acoustic reflections from different structures within the focal area can constructively and destructively interfere, producing "specle" in the ultrasonic image. Conventional transducers may also suffer from poor resolution at distances near the transducer surface (the so-called "near field"), caused by interference resulting from pathlength differences between the constituent parts of the ultrasonic beam. A further source of error results from side lobes of the ultrasonic transmit-receive system. Obviously, echoes from a weak sidelobe can obliterate the echoes from weakly reflective structures in the main beam.
Many transducer systems have been produced which have been able to improve the resolution of ultrasonic images, with a variety of approaches having been taken. In U.S. Pat. Nos. 4,106,348 to Auphan, 4,281,550 to Ericson, 3,911,730 to Niklas, and 3,881,466 to Wilcox, for example, a number of transducer elements are excited in common in order to produce a simple scanned line. Other patents, for example U.S. Pat. No. 3,820,387 to Grabendorfer, employ different sets of transducers for the transmit and receive function. Still others improve the resolution of the ultrasonic image at different distances by the provision of "dynamic focusing", whereby the number of transducers addressed is varied across the depth of a scanned line (see, for example, U.S. Pat. No. 4,241,610 to Anderson and the references cited therein. See also R. D. Selbie et al, "The Aberdeen Phased Array: A Real-Time Ultrasonic Scanner With Dynamic Focus", Medical and Biological Engineering and Computing, May 1980, pages 335-343).
U.S. Pat. No. 4,219,846 to Auphen suggests that image resolution can be obtained through the employment of concentric circles of rectangular matrix elements selected from a transducer array. However, exhaustive switching means are required to scan this concentric selection across the face of the transducer assembly. Other patents, such as U.S. Pat. Nos. 4,112,411 to Alais and 4,145,931 to Tancrell also utilize more than one element of the array to produce a single line of the B-scan picture, and add the capability to excite these different elements in differing amplitudes and phases. However, because of their high level of electronic complexity these systems are used mainly with steered-beam types of B-scanners.
It has now been found that the main characteristics needed for a high-resolution transmit-receive system for a B-scanner; that is, good focus over a large depth of view with no side lobes, can be attained with a transducer surface vibration pattern that has a peak amplitude at its center but which decays in amplitude toward the perimeter, generally in the well-known "bell" shape. A transducer excited in such a bell-shaped, or "Gaussian" vibration pattern produces a radiation pattern which has no near field on-axis intensity fluctuations and no side lobes. And, by laws of reciprocity, such a transducer also has the same directivity pattern in its receive mode; it is primarily sensitive to acoustic waves striking its surface in the axial direction, and also sensitive to on-axis reflections off interfaces in the near field region, without the usual near-field fluctuations.
A large number of transducer designs have been produced which are capable of approximating a "Gaussian" directivity pattern; however, such designs have not been useful in improving the resolution of an electronically scanned linear transducer array, since the directivity pattern of such devices is traditionally achieved by geometry-dependent means which cannot be scanned across the surface of an ultrasonic B-scan sensor.
Typical of the transducer designs which are capable of producing a "Gaussian", or bell-shaped radiation pattern is the transducer described in the paper entitled "Theoretical Study and Experiments on Spherical Focusing Transducers With Gaussian Surface Velocity Distribution" by L. Filipczynski et al, Acustica, Vol. 28, pages 121-128, 1973, wherein one electrode of the transducer is in the shape of a rosette so that the electrical excitation decreases towards the edges of the transducer. It is also possible to reduce the electrical excitation toward the edges of a transducer by employing a convex-shaped electrode which contacts the flat transducer surface only at its center, and which is spaced further away from the transducer as the distance off center increases. On a transducer having fully-plated electrodes, a similar effect can be produced by mechanical means aimed at reducing the vibrational output away from its center, either by varying the mechanical damping across the back side of the transducer, or by applying to its front face a layer of material having varying attenuation (von Haselburg et al Ein Ultraschall-Strahler fur die Werkstoffprufung mit Verbessertem Nahfeld. Acustica, 9:359-364, 1959 ).
An alternative transducer design uses a resistive plating instead of a conductive material as one of the electrodes, whereby the surface resistivity increases as the distance away from the center is increased. The electrical excitation is then applied to the center of the transducer (von Haselberg et al, ibid). Still another approach is to plate only a central part of the transducer, relying upon electrical and mechanical fringing effects to achieve a decaying excitation function toward the edges of the transducer element (M. A. Breazeale et al, "Reply to `Radiation Pattern of Partially Electroded Piezoelectric Transducers`," J. Acoust. Soc. Am. 70(6): 1791-1793, 1981).
All of the foregoing techniques achieve a shaped acoustic field pattern by mechanical or geometric shaping of the electrode or the transducer material. In each case, only a single electrical signal connection is provided, and the desired field pattern is generated by features built into the transducer, rather than through the control of a large number of individual transducers. However, the geometry-dependent mechanical construction of such devices prevents effective linear scanning of the radiation pattern and thus prevents the effective use of a "Gaussian" or bell-shaped surface excitation function in a scanning probe device.